DISTRIBUTION OF HETEROTROPHIC BACTERIA AND THEIR POSSIBLE ROLE IN THE MINERALIZATION OF ORGANIC MATTER IN A MESOTROPHIC LAKE, YUNO-KO

抄録

1) Vertical distribution of heterotrophic bacteria in the water body of Lake Yuno-ko showed a clear seasonal variation; during the summer stagnation period, the number of heterotrophic bacteria was larger in the upper layer than in the lower layer, while during the circulation period, it was distributed uniformly in vertical direction. 2) Total plankton respiration measured with concentrated plankton gave the highest value (3.11g O_2/m^2/day) in August and the lowest (0.47g) in February. During the circulation period, the total plankton respiration was uniform vertically, while during the stagnation period, it was higher in the upper layer. 3) Respiration rate of the bacterial population in the lake water was calculated on the basis of the data on the number of bacteria and the respiratory activity of one bacterial cell, which was determined with separated and concentrated bacterial population. Percentage contribution of the bacterial respiration to the total lankton respiration was estimated as only 3 to 7. 4) From the above result, it was suggested that bacteria might play a minor role in the mineralization process occurring in the water body. The major agents participating in the mineralization in the water boyy were suggested to be the phyto-and/or zooplankton. 5) Organic matter loss from the lake water by the precipitation of particulate matter appeared to be smal ; the loss was largely attributed to plankton respiration in the warmer season, and to the outflowing river in the coleer season.